This invention concerns the type of artificial cardiac pacemaker's to be implanted in the patient's body.
At the present state of knowledge, the artificial substitution of mechanisms of generation and conduction of the electrical signals which drive the contractions of the heart depend upon a vast range of artificial pacemakers which are identifiable by initials, accepted through an international agreement, according to the different conditions of action and interaction with the spontaneous activity of the heart. The term "pacemaker" will substitute "artificial cardiac pacemaker" for the rest of this description, keeping in consideration that the above mentioned term is a registered trade-mark. The kind of pacemaker in which the ventricle's stimulation activity is driven and synchronized by the detection of the spontaneous bio-electrical atrial activity are commonly defined as physiological since the electrical impulses that effect the rate are not fixed but induced by the spontaneous atrial rate and are therefore always proportional to it. This kind of pacemaker can only be used in the cases in which the atrial activity can occur in a physiological manner and, therefore, it is only possible in 30 to 60% of the actual pacemaker wearer patients.
Thus, at present, there is a high percentage of patients needing artificial cardiac stimulation that cannot make use of a physiologically variable rate stimulation with the above mentioned technique.
It is for this reason that individuals specialized in this field have been trying for some time to achieve an implantable pacemaker that can respond to a parameter, which instead of the atrial activity parameter, can vary in relation to the physiological needs of the patient, and can be used as a reference variable for automatic and physiological regulation of the electrical impulses that effect the heart rate.
As of today, the pacemakers studied in order to solve the above mentioned problem are those we will describe below in which the stimulation rate is subject, by means of a specific algorithm, to the variations of the following functions:
the hematic PH; PA1 the body temperature; PA1 the quantity of oxygen contained in the blood; PA1 the QT interval observed in an endocardial EKG; PA1 the respiratory rate.
These methods have all had insufficient success due to the complexity and difficulty the pacemaker has in detecting the above mentioned physiological variations. The limits and faults of the above mentioned methods can be summarized in a more specific manner as will follow. As for the pacemaker with control of the hematic Ph, in addition to having the essential problem of safety and reliability which arises with any sensor that is inserted directly into the hematic flow, it was also observed that the sensor decays in too brief a period with respect to the average life of a pacemaker. As for the pacemaker controlled by the amount of O.sub.2 in the venous blood there remains the essential problem of reliability and the complexity of a lead which is inserted in the hematic flow and should function both as stimulator and oxygen sensor. Doubts and hesitations must also arise regarding the co-relation between the oxymetric parameter and the heart rate since it has not yet been thoroughly defined. As for the temperature controlled pacemaker, one must remember the limits resulting from the sensor as well as the impossibility, already proven by physiological research, of the body temperature to increase with a time constant similar to that of the heart rate during the patient's physical exercise. As for the QT controlled pacemaker, evident obstacles are found in the difficulty of detecting this data and in the possibility of other causes, deriving for instance from medical therapies of natural physiological evolution, that can determine variations in the QT parameter independently of the potential variations of the heart rate. The hypothesis, which remained such, of pacemakers sensitive to the ventilation rate was based on the detection of this parameter by means of a distorsion transducer placed in an intercostal or interdiaphragmatic area or by means of acoustic transducers placed in an intra-pleural area. Besides the difficulties presented in distinguishing the respiratory signal from that of other functions thus causing it to be unreliable, there would also be the problem of needing a specific surgical approach for setting the said sensors. Another well known attempt, described by an U.S. Pat. No. 3,593,718 is the pacemaker (external, and not implantable) driven by the ventilation rate in which the said function is detected by means of an impedance voltage converter that measures thoracic impedance variations by means of electrodes on the skin of the patient when the ventilation rate changes. The signal coming from this converter is sent directly to a continuous rate/current converter that drives an oscillator that supplies the impulses needed to send the electrical stimulation necessary for the heart, by means of a constant current source at a rate which varies automatically in relation to the ventilation rate. Such a device is impossible to achieve in practice since it lacks a unit capable of converting the voltage into rate and should operate between the first and the second converters just mentioned.
Such a unit would certainly be useless if the output signal from the first converter were to be of the sinusoidal type or perfectly repetitive but it becomes absolutely necessary, presenting quite a few problems in its realization, when one deals with physiological signals that are continuously variable both in width and periodicity, and with continuous variations of the basic line, due to continuous morphological changes and overlapping of false signals. In the U.S. patent mentioned above another difficult task, that is the problem of controlling the energy drain of the entire device, so as to be able to implant it, is not taken into consideration. As a result, one can confirm that the methods proposed up to now have not enabled the various theories to result in an easy to use, totally implantable an reliable product.